The present invention relates to the manufacture of high-purity silicon wafers used in the production of integrated circuit devices, and in particular, to a method of cleaning and oxidizing the surface of silicon wafers after lapping thereof.
In the manufacture of silicon wafers, wafer blanks are first cut from an ingot of high purity silicon. The wafers are subsequently lapped by rotating disks and a liquid-born abrasive such as an aqueous slurry of finely divided Al.sub.2 O.sub.3. After lapping, the wafers are cleaned to remove residual lapping solution and silica from the lapped surface of the wafer. The wafers are then laser marked, cleaned again with alkali, acid etched, pre-washed for annealing, annealed, and finally polished to a mirror finish. In some instances, wafers may be sandblasted and washed immediately after acid etching and before the pre-wash for annealing steps. Each of these steps is well-known to those of skilled in the art, and will not be described further except as necessary for an understanding of the present invention.
Known methods of post-lapping cleaning of the wafers use a series of washing and rinsing steps. A typical post-lapping cleaning process includes:
immersing the wafer in an ultrasonic bath; PA1 removing the wafer from the ultra-sound bath and PA1 immersing the wafer in an alkali chemical bath; PA1 rinsing the wafer with purified H.sub.2 O; PA1 immersing the wafer in three successive alkali chemical baths; PA1 rinsing the wafer in four successive baths of purified H.sub.2 O; PA1 immersing the wafer in an alkali chemical bath; PA1 rinsing the wafer with purified H.sub.2 O; and PA1 unloading and spin drying the rinsed wafers.
Each of the foregoing steps typically takes an average of about three to four minutes. Overall, the post-lap cleaning of the wafers represents a significant expense, including the consumption of significant volumes of water and alkali chemicals. Known post-lapping cleaning methods also limit the overall wafer production rate, further adding to the final cost of wafer production.
U.S. Pat. No. 5,484,748 to Suzuki et al. teaches a method for storage of wafers in an H.sub.2 O.sub.2 solution after the acid etching step discussed above. Suzuki addresses the problems attendant in the storage of wafers which have been acid etched, and either in the acid etch step or thereafter, have been treated with an acid or alkali reagent. Suzuki teaches that during processing, wafers must often be stored between processing steps, in some instances for days at a time. Suzuki teaches that the expense and difficulties associated with the storage of wafers in purified water or in clean dry air (having a degree of cleanliness of 100 or more) can be reduced by storing the wafers for up to 120 hours in a solution of between 0.01 to 30% H.sub.2 O.sub.2 at a temperature between 0-80.degree. C. Suzuki teaches that contamination of wafer surfaces which have been treated with acid or alkali cleaning or etching reagents may thus be avoided while at the same time providing greater flexibility in the manufacturing process. Suzuki does not address, however, the lengthy and expensive process of post-lapping cleaning and passivation of wafers, which represents a significant cost factor and production rate-limiting step.
A need therefore remains for a post-lap cleaning method which is simpler and faster than the current cleaning methods of the prior art, and one which reduces the required number and amount of post lapping cleaning reagents.